Ionization-type fire sensor

ABSTRACT

To permit free entry of ambient air, without clogging, dust deposits, and for easy cleaning of the sensor, the housing for the sensor is formed in at least two parts separated from each other by an essentially continuous ring-shaped slit, the parts of the housing in the vicinity of the slit being formed to provide a labyrinth path for passage of air through the slit into the chamber, in which the direction of air flow is deflected from an essentially straight path; one of the electrodes of the sensor is an air-pervious grid or mesh located to prevent mechanical access to the other electrode.

United States Patent 11 1 1111 3,908,957

Schiitt 1 Sept. 30, 1975 IONlZATlON-TYPE FIRE SENSOR 3,662.177 5/1972 Sasaki .1 250/384 3,710,110 1/1973 Lampart 250/38l [75] Inventor: schut" ,Zonlkerbcrg- 3,731,093 5/1973 Scheidweiler 250/384 swltzerhlhd 3,767,917 10/1973 Lampart 250/384 [73] Assignee: Cerberus AG, Mannedorf,

Swit l d Primary E.\aminer-Harold A. Dixon Attorney, Agent, or FirmFlynn & Frishauf [22] F1led: Apr. 4, 1974 [21] Appl. No.: 457,841 [57] ABSTRACT To permit free entry of ambient air, without clogging, [30] Foreign Application P i it D t dust deposits, and for easy cleaning of the sensor, the Apr 17 1973 Switzerland 5519 73 housing for the Sensor is formed in at least Parts separated from each other by an essentially continu- 52 U.S. c1. 250/384; 250/385- 340/237 s Ous ring-Shaped Shh the Parts of the housing in the 51 1111. c1. GOlT 1/18- @0813 21/00 cihhy of the Slit being formed to Provide a labyrinth 58 Field 01 Search 250/374 3 84 331 385 Path Passage of air through the slit into the cham- 3 ber, in which the direction of air flow is deflected from an essentially straight path; one of the electrodes [56] References Cited of the sensor is an air-pervious grid or mesh located to UNITED STATES PATENTS prevent mechanical access to the other electrode.

3,319 069 5/1967 Vassil 250/381 17 Claims, 2 Drawing Figures US. Patent Sept. 30,1975 3,908,957

14 c EA AT Fig.2

' IONIZATION-TYPE FIRE SENSOR CROSS REFERENCE TO RELATED APPLICATIONS U.S. Ser. No. 374,795 and U.S. Ser. No. 374,310, both filed June 28,1973.

The present invention relates to an ionization-type fire sensor in which a housing is provided which has openings to permit the entry of ambient air to an ionization chamber, containing two electrodes and a radioactive source to provide an ion current. The ion current changes when smoke or fire aerosols are present in the ambient air.

Ionization-type fire sensors must meet many, simultaneously applicable requirements. Their sensitivity should be high, that is, the ion current should change upon presence of even a very small smoke or fire aerosol concentration within the ionization chamber; the change in the ion current should be marked, and high. Using a low field strength results in high change in ion current upon presence of smoke, or fire aerosols.

Ionization fire sensors are quite sensitive to air current. It is, therefore, necessary to slow any air stream within the ionization chamber by means of deflecting elements, or the like. Dust which is carried by air flowing into the chamber is apt to deposit at the air deflecting elements, at which air is backed up by deflecting baffles, so that certain zones of the fire sensor housing relatively quickly are coated with dust and dirt. Such coating is particularly noticeable in the vicinity of the openings to the housings. Known ionization fire sensors therefore require frequent cleaning of the housing to remove deposited dust and dirt. This requires a good deal of labor and is expensive.

Ionization-type fire sensors contain sensitive parts; upon opening of the housing, for example for cleaning, the danger is everpresent that such sensitive parts are damaged. Low voltage ionization fire sensors which use a field effect transistor in their sensing circuit are particularly subject to damage; touching the electrode of the sensor which is connected to the control electrode of the field effect transistor may lead to destruction of the transistor. It has, therefore, been found necessary to protect the field effect transistor upon opening of the housing by suitable circuitry, for'example by protecting switches, in order to avoid destruction thereof. This circuitry is additional to the regular sensor circuitry and hence adds costs.

It is an object of the present invention to provide a sensor which is so designed that it is less subject to contamination by dirt and dust, can be cleaned simply, and is so arranged that the danger of damage of components thereof by touching sensitive elements is essentially avoided, without decreasing the sensitivity of the sensor, or increasing the tendency to give false alarms based on air currents.

SUBJECT MATTER OF THE PRESENT INVENTION Briefly, the housing is formed in at least two portions or parts, so arranged that one portion thereof, at least, can beremoved. The'openings forthe ingress of air are a ring-shaped slit, whose circumferential extent is interrupted as little as possible (at best by holding clips, screws or the like). Means'are provided to brake'and deflect air passing through the slit; these means are, preferably, a labyrinth arrangement interiorly of the slit. One of the electrodes is formed as an air-pervious grid or mesh and is so located within the housing that it protects the other electrode which, then, can be connected to the control electrode of a field effect transistor (FET).

, The'invention will be described by way of example with reference to the accompanying drawings, wherein:

FIG 1 is a longitudinal sectional view through one embodiment of a sensor which, in plan view, may be circular; v

'and FIG. '2 is a highly schematic longitudinal view of another embodiment, in which the opening to the housing is differently arranged.

The ionization fire sensor of FIG. 1 has a socket part 1 and a sensor part2. Socket 1 is formed with a lateral opening in which an indicator lamp 3, for example a light-emitting diode, is located.

The sensor part 2 has a housing which comprises two portions; one is a cylindrical or slightly conical housing portion 4 and a bottom cover portion 5. Housing portions 4 and S are separated by a ring-shaped slit 13 which is, preferably, no more than /2 cm wide. Air can pass through slit 13 into the interior of the housing. The inside of the cover 5 has an upwardly projecting baffle 12. The height of the baffle 12 is, preferably, at least l cm, and it extends cylindrically around the inside of the slit 13 to deflect air entering into the sensor upwardly in vertical direction.

The distance of the baffle 12 from housing may be approximately in the order of the size of the width'of the slit l3, somewhat greater or somewhat less. It need not be always of the same width, but can'in'crease in size towards the top, for example if the baffle 12 is cylindrical and the housing 4 is conical (FIG. l The distance towards the upper opening may also decrease, for example if the baffle 12 is formed conically with a greater conical angle than the conical angle of housing 4. The bottom cover 5 and the baffle 12, which may be integral therewith, for example by being welded or soldered to the cover 5, are held or secured to the housing 4 by holding elements connecting the cover 5 and the housing 4 in such a manner that the ring-shaped labyrinth slit permitting ingress of air is interrupted as littleas possible. Cover 5. may be permanently secured to the housing shield 4, for example by a few wire pins connecting the housing 4 to the cover 5; or it may be removably attached, for example by three equidistantly spaced circumferential screws-of which one is shown at 41, similar to the holding arrangement for lamp globes in lamp sockets. Any other suitable attachment of the housing bottom 5 to the surrounding shield 4 may be used, rigidly or removably interconnecting these two parts, if care is taken that the slit 13, or the inner por tion of the slit is interrupted as little as possible; The diameter of the cover 5 may be larger than the diameter of the housing 4.

I The air inlet system has particularly advantageous characteristics if the following conditions are met:

a. the height H of the baffle 12 should be at least twice as high as the width B of the slit 13;

b. the maximum dimension of the distance A between the baffle 1'2 and the housing 4 should be at most twice the dimension of the width B of the slit 13;

c. the depthT of the slit 13 should be at least half the width B ofthe slit 13.

In one form of the invention which has been found suitable, and which is given byway of example, the

width B of the slit 13 was 3 mm; the height H of the baffle 12 was 9.5 mm; the maximum distance A between the baffle 12 and the housing 4 was 3.5 mm; the depth T of the slit 13 was 5 mm. The diameter of the housing as the root of the shield 4 was 70 mm.

The relationship of the above dimensions, to each other, and particularly the values of H, T and A with respect to the width B are of importance, rather than the absolute dimensions.

Within housing 4, a carrier plate 6 made of insulating material is secured. Carrier plate 6 holds all other elements of the sensor. A central bore within the carrier plate has a metallic bolt 7 located therein, carrying a disk-shaped electrode 8 on which a radioactive source 9, such as a preparation including Americium 241 is located. A wire mesh, entirely surrounding electrode 8 and formed to be cup-shaped is located at the bottomside of carrier plate 6 to form counter electrode 11. Wire mesh electrode 11 entirely surrounds the inner electrode 8 and prevents contact with the inner electrode 8, for example accidentally. The center electrode 8 and the counter electrode 11 define, between themselves, the ionization chamber 10. An electrical circuit, which may, for example, be an integrated circuit or otherwise an entirely encapsulated circuit 14, is located above the carrier plate 6. Electrical circuit 14 preferably includes an FET input transistor, and evaluates changes in ionization current within the chamber 10. A chamber, entirely or almost entirely separated from outer atmosphere can further be provided in order to form a reference ionization chamber connected, as known, in series to the ionization chamber 10. The carrier plate 6 is preferably secured to the socket part 1, for example by holding bracket 42 molded into the carrier plate 6 or otherwise attached thereto, which brackets, in turn, may be spot-welded, or suitably connected to the socket part I, to securely hold the plate 6 to the socket part 1. These holding brackets 42 may, also, be removably located within the socket, so that the carrier plate 6 can be removed as a unit with the housing from the socket. In this case, suitable connectors must be provided from the electrical circuit 14 to a connecting cable 14a which, in turn, connects to outside terminals at the side of the sensor, as schematically shown.

In order to keep dust and dirt from the interior of the ionization chamber 10, the housing portion, or shield 4 and the bottom cap 5 are made of metal and placed at a voltage which is different from that of the voltage applied to the mesh or grid electrode 1 1. A connection, therefore, is brought out from electrode 11 through the insulating plate 6 to terminal 11 which, preferably, is connected to a source of positive voltage. Alternatively, the entire housing may be made of nonconductive material, so that dust particles will precipitate on the housing, due to static charge of the material, rather than being carried into the ionization chamber.

In a preferred form, the cover 5 of the housing is removable from the shield part 4 in order to facilitate cleaning.

The shield part 4 is retained on the plate 6 by suitable means. for example by slight depressions formed circumferentially around the plate 6, in which punched-in resilient projections, formed in shield part 4, can engage. Thus, shield part 4 can be removed from the plate 6, as a unit with the bottom cover 5, if desired; for as sembly, shield part 4 is merely pushed on the plate 6 until the snap openings engage. Alternatively, the plate 6 may be formed with a ring groove, in which spring fingers engage, or projections engaging bayonet-holes in the shield 4 which are then covered internally by the plate 6, or by other suitable holding and separable engagement arrangements.

The construction of the ionization-type sensor provides a slit-ring-shaped opening 13 in which the air is braked and dammed immediately behind the opening by the baffle 12, and air flow is deflected in vertical direction. The deflection is so arranged that the air flow is tangential to the grid or mesh 11. The relatively heavy dust particles will largely precipitate in the damming region; the much lighter smoke particles will, hardly, precipitate however and will be carried into the chamber 10. Dust particles, therefore, will precipitate on the housing, or at least in the immediate vicinity of the entry slit l3 and will not penetrate into the ionization chamber 10. Ingress of smoke is, however, hardly impeded. By removing housing 4 from the carrier plate 6, or even by merely removing the bottom cap 5 from the housing 4, dust which has precipitated can be easily removed, thus permitting easy cleaning of the sensor. Rather than screws 41, resilient snaps or bayonet-slits in the housing 4, engaging bent-over lugs projecting from bottom cap 5 or the baffle 12 may be used for simple disassembly. The mesh or grid 11 protects the center electrode 8 connected to the control electrode of an PET in the circuit 14 when the housing 4, or the cover 5 has been removed from the sensor. Thus, upon cleaning, damage to the sensitive electrical circuit and particularly to the FET is prevented.

In the embodiment of FIG. 2, the housing 21 is separated by a ring-shaped slit 22 from an attachment surface 27. The measuring ionization chamber 20 has a central electrode 23 and an outer electrode 24 which is formed as a fine mesh grid. The housing 21 is removable from the base plate 26 which, preferably, is of insulating material, such as a plastic block. The attachment of the housing 21 to the plastic block 26 may, for example, be by means of pins molded into block 26 and projecting radially outwardly, passing through holes and slits formed in the housing 21, in form of a bayonet connection. The air which passes into the chamber 20 is dammed and deflected upon entry through the slit 22 by the side surfaces 25 of the insulating block 26. This construction, also, provides for damming and deflection of air passing into the sensor. Dust will usually mostly precipitate at the inner wall of the housing 21. Mesh electrode 24 rejects dust particles, without preventing passage of air, carrying smoke or fire aerosols along. The mesh or grid electrode 24 further protects the field effect transistor connected to center electrode 23. The electrical circuit is preferably contained within a hollowed portion of the block 26, and then sealed therein by means of a casting compound. The electrical connections have been omitted from FIG. 2.

The metal portions of the housing if metal is used for example socket l, are preferably grounded. If the shield 4 (FIG. 1 is metal, a metallic connection is made between the socket l and the shield 4, for example by engagement of the socket l with a depressed resilient projection lb formed on a cross plate la of the socket l. The block or plate 6 likewise is attached to cross plate la, which can also support such releasable plug-socket. connections for the electrical circuit as may be needed. The luminescent diode 3 is shown connected in circuit with the sensor.

For circuits for use with a sensor, reference is made to copending applications Ser. Nos. 374,310 and 374,795, both filed June 28, 1973, and assigned to the assignee of the present application.

Various changes and modifications may be made within the scope of the inventive concept.

I claim:

1. Ionization-type fire sensor having a socket (l, 25), an ion source (9) an inner and an outer electrode (8, 23; 11, 24), the outer electrode being located and shaped to define an ionization chamber in which ion current between said electrodes, upon application of an electric field, changes upon presence of fire or smoke aerosols within the chamber, wherein the outer electrode comprises an essential continuous, air-pervious grid or mesh (11; 24), shaped to at least partly surround the inner electrode to prevent mechanical access to the inner electrode (8, 23) and hence accidental contact with the inner electrode; and

a housing coupled to said socket surrounding the electrodes and in part spaced therefrom, and comprising at least two portions (4, 5; 21, 27), said portions being separated from each other by an essentially continuous ring-shaped slit, one portion being essentially cylindrical and forming a sleeve-like shield for the outer electrode, the other portion extending transversely to the axis of the sleeve-like cylindrical shield, said portion being spaced from the end of the shield to form the slit, said housing being formed with a baffle portion having a wall portion extending in a direction parallel to the inner wall of the cylindrical sleeve-like shield, said portions thereby, and conjointly defining a path for passage of air through the slit into the chamber, in which the direction of air flow is deflected from an essentially straight path diametrically with respect to the cylindrical sleeve-like portion to essentially tangential direction with respect to said grid, or mesh electrode (11, 24) and in which at least one of said portions is removable from the socket.

2. Sensor according to claim 1, wherein an evaluation circuit including a field effect transistor (FET) (14) is provided, the inner electrode (8, 23) being connected to an input electrode of the PET.

3. Sensor according to claim 1, wherein the housing is of metal, and grounded.

4. Sensor according 0 claim 1, wherein the housing is formed of electrically non-conductive material.

5. Sensor according to claim 1, wherein the socket includes an essentially horizontal plate-like support block of insulating material (6, 26), said electrodes being supported from said support block, and said grid or mesh-shaped electrode being shaped in form of an inverted cup depending from said plate-like block to define within said cup the ionization chamber.

6. Sensor according to claim 5, wherein the sleevelike cylindrical shield is removably attached to the support block (6, 26) of insulating material.

7. Sensor according to claim 1, wherein the housing is formed as a cap (21), an attachment plate (27) is provided and forming said other portion and a portion of the socket, the slit (22) being formed by a ringshaped gap between the cap (21) and said attachment plate (27).

8. Sensor according to claim 7, wherein the sensor includes a block (26) of insulating material located inside said cap (21) secured to said attachment plate and formed with said wall portions (25) located behind the gap (22) and forming the deflecting surfaces for air passing through said path.

9. Sensor according to claim 8, wherein the mesh or grid electrode (24) is secured to the block (26) of insulating material.

10. Sensor according to claim 1, wherein the other portion of the housing comprises a cover (5), the slit and hence the entry opening for air to the ionization chamber (10) being formed between the end of the sleeve-like portion (4) and the cover.

11. Sensor according to claim 10, wherein the projecting baffle portion comprises a projecting baffle (12) formed on the inner side of the cover (5) to deflect air passing into the slit.

12. Sensor according to claim 10, wherein the cover (5) is removable and separable from the sleeve (4).

13. Sensor according to claim 10, wherein the slit (13) between the sleeve (4) and the cover (5) has a width of about /2 cm or less.

14. Sensor according to claim 11, wherein the projecting baffle is an essentially cylindrical, or slightly conical projecting baffle ring (12).

15. Sensor according to claim 11, wherein the projecting baffle (12) has a height of at least 1 cm.

16. Sensor according to claim 1 1, wherein the height (H) of the projecting baffle (12) is at least twice the width (B) of the slit (l3).

l7. Sensor according to claim 16,

wherein the maximum distance (A) of the baffle (12) from the interior of the housing sleeve (4) is at most twice the width (B) of the slit (13), and the distance (T) between the baffle (12) and the housing sleeve (4) of the slit is at least half the width (B) of the slit. 

1. Ionization-type fire sensor having a socket (1, 25), an ion source (9) an inner and an outer electrode (8, 23; 11, 24), the outer electrode being located and shaped to define an ionization chamber in which ion current between said electrodes, upon application of an electric field, changes upon presence of fire or smoke aerosols within the chamber, wherein the outer electrode comprises an essentiall continuous, airpervious grid or mesh (11; 24), shaped to at least partly surround the inner electrode to prevent mechanical acceSs to the inner electrode (8, 23) and hence accidental contact with the inner electrode; and a housing coupled to said socket surrounding the electrodes and in part spaced therefrom, and comprising at least two portions (4, 5; 21, 27), said portions being separated from each other by an essentially continuous ring-shaped slit, one portion being essentially cylindrical and forming a sleeve-like shield for the outer electrode, the other portion extending transversely to the axis of the sleeve-like cylindrical shield, said portion being spaced from the end of the shield to form the slit, said housing being formed with a baffle portion having a wall portion extending in a direction parallel to the inner wall of the cylindrical sleeve-like shield, said portions thereby, and conjointly defining a path for passage of air through the slit into the chamber, in which the direction of air flow is deflected from an essentially straight path diametrically with respect to the cylindrical sleeve-like portion to essentially tangential direction with respect to said grid, or mesh electrode (11, 24) and in which at least one of said portions is removable from the socket.
 2. Sensor according to claim 1, wherein an evaluation circuit including a field effect transistor (FET) (14) is provided, the inner electrode (8, 23) being connected to an input electrode of the FET.
 3. Sensor according to claim 1, wherein the housing is of metal, and grounded.
 4. Sensor according to claim 1, wherein the housing is formed of electrically non-conductive material.
 5. Sensor according to claim 1, wherein the socket includes an essentially horizontal plate-like support block of insulating material (6, 26), said electrodes being supported from said support block, and said grid or mesh-shaped electrode being shaped in form of an inverted cup depending from said plate-like block to define within said cup the ionization chamber.
 6. Sensor according to claim 5, wherein the sleeve-like cylindrical shield is removably attached to the support block (6, 26) of insulating material.
 7. Sensor according to claim 1, wherein the housing is formed as a cap (21), an attachment plate (27) is provided and forming said other portion and a portion of the socket, the slit (22) being formed by a ring-shaped gap between the cap (21) and said attachment plate (27).
 8. Sensor according to claim 7, wherein the sensor includes a block (26) of insulating material located inside said cap (21) secured to said attachment plate and formed with said wall portions (25) located behind the gap (22) and forming the deflecting surfaces for air passing through said path.
 9. Sensor according to claim 8, wherein the mesh or grid electrode (24) is secured to the block (26) of insulating material.
 10. Sensor according to claim 1, wherein the other portion of the housing comprises a cover (5), the slit and hence the entry opening for air to the ionization chamber (10) being formed between the end of the sleeve-like portion (4) and the cover.
 11. Sensor according to claim 10, wherein the projecting baffle portion comprises a projecting baffle (12) formed on the inner side of the cover (5) to deflect air passing into the slit.
 12. Sensor according to claim 10, wherein the cover (5) is removable and separable from the sleeve (4).
 13. Sensor according to claim 10, wherein the slit (13) between the sleeve (4) and the cover (5) has a width of about 1/2 cm or less.
 14. Sensor according to claim 11, wherein the projecting baffle is an essentially cylindrical, or slightly conical projecting baffle ring (12).
 15. Sensor according to claim 11, wherein the projecting baffle (12) has a height of at least 1 cm.
 16. Sensor according to claim 11, wherein the height (H) of the projecting baffle (12) is at least twice the width (B) of the slit (13).
 17. Sensor according to claim 16, wherein the maximum distance (A) of the baffle (12) from the interior of the housing sleevE (4) is at most twice the width (B) of the slit (13), and the distance (T) between the baffle (12) and the housing sleeve (4) of the slit is at least half the width (B) of the slit. 